(1) Field of the Invention
The present invention relates to a wire disconnection diagnosis apparatus of a fuel injector drive circuit used for an electronically controlled fuel injector of an internal combustion engine.
(2) Description of the Related Art
In an electronically controlled fuel injector of an internal combustion engine, an electromagnetic system fuel injector is provided at each cylinder, or commonly to all the cylinders, in the engine intake system.
One end of a solenoid of this fuel injector is connected to a power source, and the other end is grounded via transistors. The transistors are electrified by a drive pulse signal (Ti pulse) output in synchronization with the revolutions of engine, while the solenoid is thus magnetized and the needle valve of the fuel injector is then opened for the fuel injection.
Further, during the fall of the Ti pulse, the transistors are turned off and the solenoid is demagnetized, whereby the needle valve is opened to stop the fuel injection. During this period, however, a counter electromotive voltage is generated at the solenoid, after an elapse of a predetermined time from the fall of the Ti pulse.
Here, from the presence/absence of a counter electromotive voltage generated at the solenoid when the electric power is shut off, a wire disconnection in the fuel injector drive circuit is diagnosed (see Japanese Unexamined Patent Publication (Kokai) No. 2-130261).
Namely, the counter electromotive voltage at the solenoid is taken out and input to a discharge circuit via an amplitude circuit and a waveform shaping circuit. This discharge circuit is composed of a capacitor charged via a diode and a transistor connected in parallel with the capacitor for discharging the capacitor during the supply of electric power.
The discharge potential (the terminal voltage at the capacitor) of the charge/discharge circuit is determined by a microcomputer during the generation of a reference signal output at a predetermined crank angle by a crank angle sensor, and a wire disconnection is detected when the potential is lower than a predetermined value. Further, by outputting a clear signal to turn on the transistors in the charge/discharge circuit immediately after this determination, the capacitor is discharged and is ready for the next charge.
According to this type of wire disconnection diagnosis apparatus, the waveform shaping is provided only at the portion corresponding to the counter electromotive voltage from the drive waveform signals generated at the solenoid, to keep the capacitor charged, and a wire disconnection in fuel injector drive circuit can be diagnosed by determining whether the terminal voltage at the capacitor is at a high or low level during the generation of reference signals from the crank angle sensor. Namely, the absence of a wire disconnection (OK) is diagnosed when the terminal voltage at the capacitor is at a high level, and the presence of a wire disconnection (NG) is diagnosed when the terminal voltage of capacitor is at a low level.
In the wire disconnection diagnosis apparatus of this type of fuel injector drive circuit, however, a predetermined time must pass from the start to the end of the discharge, and during the discharge, the clear signal must be maintained at a high level for this predetermined time, and thus the time must be controlled by a software timer, to maintain the clear signal at a high level, and therefore, a problem exists in that a load on the software is increased by that portion with a resultant overloading of the software.
The terminal voltage of capacitor is determined by the generation of reference signals from the crank angle sensor, but when the fuel injection timing deviates to a delay side due to the generation of reference signals during the generation of a counter electromotive voltage, and the capacitor has not yet been charged, although a determination can be made, a problem exists in that an erroneous determination may be made at that time.